Due to the high costs associated with manufacturing integrated circuits (ICs), manufacturers typically perform testing at the wafer level, before further processing and packaging, and more importantly, before a customer becomes dissatisfied. A typical conventional testing method is as follows. A wafer is mounted on a chuck and held in place through suction. Probes are brought into contact with the circuit devices on the side of the wafer opposite to the chuck. The circuits are tested, sometimes at high power levels (on the order of several hundred watts per square centimeter), for functionality, power integrity, reliability and speed.
High power levels during testing will cause the devices to overheat which may result in inaccurate test results or damage to the device or probes. As the temperature increases, the device may tend to draw even more current, which results in further temperature increase, also known as thermal runaway.
Commonly, the chuck is configured to act as a heater or heat sink, being composed of copper, and having channels or a manifold to carry a temperature-regulating fluid. Effectively, the fluid regulates the temperature of the chuck, and the chuck regulates the temperature of the wafer. Chucks may have heating or thermoelectric elements to further control the temperature.
However, as the power of the device increases, the device temperature will increase above the chuck temperature due to the thermal interface resistance between the device and chuck. Improvements to the chuck to reduce the thermal interface resistance have been attempted, such as optimizing the surface finish, hardness and thermal conductivity of the chuck or the introduction of a high conductivity gas (helium) between the wafer and the chuck.
Further improvements to the interface resistance have been attempted by introducing a thermal interface fluid between the wafer and chuck at atmospheric pressure and simultaneously drawing the fluid away by vacuum. Use of water or aqueous solutions as a thermal interface fluid is advantageous due to its high thermal conductivity. Disadvantages of water are the potential for corrosion or electrical shorts if there is a leak. Various dielectric fluids are advantageous because thermal conductivity is higher than helium, there is minimal risk of corrosion or electrical shorts and any small amount of liquid left on the wafer evaporates without leaving a residue.
Such methods are unfavorable, though. First, because the wafer is held on to the chuck by suction, and the thermal interface fluid is drawn away from the interface at reduced pressure, such a fluid will often vaporize at reduced pressure, reducing the efficiency of the vacuum, and frequently escaping from the system during use. Even if the fluid is cooled after being drawn away from the interface, in order to condense the vapor back into a liquid, a substantial amount of vapor may still be exhausted. The preferred dielectric thermal interface fluids, which readily evaporate and leave little residue, typically have a low vapor pressure and are both expensive and not environmentally friendly. Therefore, any such exhaust is to be avoided.
Second, such a system is necessarily so complex that it is not only initially costly, but also expensive to maintain and prone to failure. Third, the thermal conductivity of a vapor is lower than that of a liquid. Therefore, the more the fluid at the interface is composed of a vapor (and the less of a liquid), the worse the performance of the fluid as a thermal interface.